In recent years, CO2 emissions regulations have become stricter due to the increasing concern over environmental problems. The automobile industry has been confronted with the challenge of enhancing fuel efficiency by reducing the weight of automobile bodies. Thus, thickness reduction has been pursued by adopting high strength steel sheets for automobile components. In detail, steel sheets having a tensile strength of 590 MPa or more have come to be used for the manufacturing of components that used to be made from steel sheets with a tensile strength of 270 to 440 MPa.
Steel sheets with 590 MPa or higher tensile strength are required to be excellent in workability such as elongation and stretch-flange-formability (flange forming property) from the viewpoint of formability, and are also required to have high crash absorption energy characteristics. Increasing the yield ratio is effective for enhancing crash absorption energy characteristics, and makes it possible for the steel to absorb crash energy efficiently even with small deformation.
Steel sheets may be strengthened to achieve a tensile strength of not less than 590 MPa by way of the hardening of ferrite that is the mother phase or by utilizing hard phases such as martensite and non-recrystallized ferrite. Methods associated with the hardening of ferrite include solid solution strengthening by the addition of such elements as Si and Mn, and precipitation strengthening by the addition of carbide-forming elements such as Nb and Ti. For example, Patent Literatures 1 to 3 propose steel sheets obtained through precipitation strengthening by the addition of Nb and Ti.
On the other hand, the utilization of hard phases is described in Patent Literature 4, which discloses high strength steel sheets with excellent stretch-flange-formability and anti-crash property in which the main phase is a ferrite phase, the second phase is composed of a martensite phase, the maximum grain diameter of the martensite phase is not more than 2 μm, and the area fraction of the martensite phase is not less than 5%. Patent Literature 5 discloses high strength cold rolled steel sheets with excellent workability and anti-crash property which are obtained through Nb and Ti precipitation strengthening and further contain non-recrystallized ferrite and pearlite. Methods for manufacturing such high strength cold rolled steel sheets are also disclosed in the same literature. Further, techniques are proposed (for example, Patent Literatures 6 and 7) to enhance both the strength and the stretch-flange-formability of steel sheets which have a microstructure including ferrite and pearlite.
[PTL 1] Japanese Patent No. 2688384
[PTL 2] Japanese Unexamined Patent Application Publication No. 2008-174776
[PTL 3] Japanese Unexamined Patent Application Publication No. 2009-235441
[PTL 4] Japanese Patent No. 3887235
[PTL 5] Japanese Unexamined Patent Application Publication No. 2009-185355
[PTL 6] Japanese Patent No. 4662175
[PTL 7] Japanese Patent No. 4696870